The protester collapsed at around 7.30pm on Wednesday as violent clashes with police continued into the night near the Bank of England.
However, as officers went to the man’s aid, they were pelted with bottles and other missiles, forcing them to retreat.
Police medics broke the cordon and tried to resuscitate the man before he was rushed by ambulance to hospital where he was later pronounced dead.
The Independent Police Complaints Commission has been informed about the incident, Scotland Yard said.
The death came after a day of protests in which a Royal Bank of Scotland branch became a battleground as masked and hooded thugs smashed windows and ransacked the building.
In front of a cheering mob, they sprayed the words “thieves” and “scum” on the walls before riot police moved in. Three people have since been charged with possession of a bladed weapon and one with assault.
It was one of a number of sporadic clashes as around 6,000 people marched on the City and Trafalgar Square to demonstrate against grievances ranging from “greedy” bankers to climate change and the wars in Iraq and Afghanistan.
Fears of organised riots – which had led to an unprecedented 5,000 police officers being deployed – proved unfounded as the vast majority of demonstrators refused to be lead by a small cadre of around 40 “black bloc” anarchists intent on stirring up trouble.
Bankers defied warnings that they could be targeted and arrived for work as usual in the morning, though many had dressed down to avoid drawing attention to themselves.
The protests outside the Bank of England, organised by a group called G20 Meltdown, started peacefully shortly before midday as four groups, marching behind four “horsemen of the apocalypse” converged on the City from four railway stations.
At first, jazz bands, jugglers and drummers lent a carnival atmosphere to the gathering, but within an hour, anger at the collapse of the financial system turned to violence – with RBS the inevitable target.
One group held aloft an effigy of Sir Fred Goodwin, the disgraced former chief executive of RBS, whose home in Edinburgh was vandalised earlier this month, while others chanted “build a bonfire, put the bankers on the top”.
Scuffles broke out as a few individual protesters tried to rush police lines, throwing purple smoke bombs and pink paint at officers, who retreated over barriers.
One crop-haired protester, whose head was cut in a clash with police, repeatedly tried to goad officers by shouting at them as blood poured down his face before he was eventually arrested.
A policeman had to be treated for a head injury after a hooded thug hurled a metal stick at him.
Just before 1pm, dozens of black hooded anarchists set their sights on a branch of the Royal Bank of Scotland on Threadneedle Street which was closed but had not been boarded up.
After forcing their way past police, they smashed the windows with a metal pole and climbed inside, briefly starting a small fire before throwing a printer, paper and other office equipment outside and stealing a computer hard drive before police dressed in riot gear cleared the building.
For every act of savagery there was also a surreal counterbalance, with one protester dressed as a huge rabbit being dragged away by police.
The man who died collapsed in St Michael’s Alley, near Birchin Lane just off Cornhill, and another protester called police to warn them that he was not breathing.
Police medics managed to get the man to a safe area where they tried to resuscitate him before an ambulance arrived and rushed him to hospital, where he died at around 8pm.
A Scotland Yard spokesman said: “The officers took the decision to move him as during this time a number of missiles – believed to be bottles – were being thrown at them.”
Elsewhere around 2,000 anti-war protesters marched peacefully from the US Embassy in Grosvenor Square to Trafalgar Square, where they were addressed by Tony Benn.
Stuart Fraser, of the City of London Corporation, praised the police for achieving “a very difficult balancing act” by allowing protesters to march but also keeping the City open for business.